I am able to reproduce the problem on my Mountain Lion machine—that is, I do see the message that "libWebKitSystemInterfaceMavericks.a: not in gzip format". The port should be fixed to not try to extract this file, by listing all the files that should be extracted in extract.only. However, on my system it is just a warning message; it does not stop the build.
I think the reason why it turned into an error on your system is that you have /sw/bin/gnutar, and it's a different version than the one that comes with OS X. /sw is the directory where Fink traditionally installs software. Fink and MacPorts serve the same purpose—to let you install software packages—but they can interfere with one another, as indeed they do here: MacPorts has inadvertently detected and is using Fink's copy of gnutar.
The solution to that is to pick a single package manager—MacPorts or Fink—and uninstall the other. We do not support using multiple package managers on the same system.
If you choose to stay with MacPorts and have removed Fink, you'll have to reinstall MacPorts base (you can use the installer package on our web site, and reinstalling base will not adversely affect your installed ports) so that it no longer users Fink's gnutar. You may also have to reinstall some of the ports you have installed, which may have suffered the same problem (for example if a port X complains of missing libraries or programs beneath /sw, rebuild X with "sudo port -n upgrade --force X").
